Mix the cornflour and soy sauce to make a smooth paste, then add the water, sugar and malt vinegar. Set aside.
Heat half the oil in a wok over a high heat until the oil starts to shimmer. Add the chicken strips and season with salt and white pepper. Stir-fry for 3–4 minutes until golden, then remove the chicken with a slotted spoon and set aside.
Return the wok to the heat and pour in the remaining oil. Add the ginger, chilli and leek and stir-fry over a medium heat for 3–4 minutes, until the leeks have started to soften. Stir in the asparagus and cook for 1 minute.
Tip the chicken back into the wok and cook for 1 minute. Pour in the cornflour mixture and cook, stirring, until it becomes a thick and velvety sauce. Serve with rice, if liked.